1. Field of the Invention
This invention relates to the field of signal correction in a communication system, and, more particularly, to precorrection of an information carrying input signal to a non-linear amplifier.
2. Description of the Prior Art
Correction for amplifiers has attracted a significant amount of interest recently, mainly due to increasingly strict requirements for spectral purity and demands for increased efficiency. With digital signal processing (DSP) circuits becoming less expensive and more powerful, predistortion of an information carrying signal has become a popular means of meeting these requirements.
Predistortion is relatively straightforward in a memoryless system. In a memoryless system, the output of the system at a given time depends solely on the input to the system at the given time. A solid first approximation of the characteristics of a memoryless amplifier can be provided by the transfer curves of the amplifier. Predistortion in such systems is well known in the art.
Most practical amplifiers, however, have memory, in the sense that prior inputs have an effect on the present output. Such a system can be only roughly approximated by memoryless system. One approach for dealing with non-linear systems with memory involves the Volterra series. When dealing with high power amplifiers, however, the limitations of the Volterra series become apparent; under present theory, a finite Volterra series can not handle the saturation that plagues any practical amplifier.
Efforts have been made to work around this limitation, but these efforts have achieved only limited success. One system was described at the Thirty-Second Annual Asilomar Conference on Signals, Systems, and Computing under the title xe2x80x9cExact and pth Order Equalization and Linearization of Recursive Polynomial Systems.xe2x80x9d The system is shown to be capable of approximating non-linear systems with memory using the Volterra series. The derived model, however, is effective only for systems in which the amplifier model is of general minimum phase type. By xe2x80x9cgeneral phase typexe2x80x9d, it is meant that the error is leading the signal, much as in a linear system that is non-minimum phase. Such a limitation is impractical for many applications.
To overcome the limitations discussed above, an apparatus is provided for precorrection of non-linear amplifiers, in particular those with memory. To this end, a method is disclosed for predistorting an input signal to a non-linear amplifier. An appropriate precorrection is determined for the input signal over a series of iterative stages according to an amplifier model. The amplifier model includes a limiting function that clips the input signal to reduce error due to amplifier saturation. The determined precorrection is then applied to the delayed input signal.
In accordance with another aspect of the invention, a method is disclosed for predistorting an input signal to a non-linear amplifier. An appropriate precorrection is determined for the input signal over a series of iterative stages, according to an amplifier model. The input signal is delayed between iterative stages. The determined precorrection is then applied to the delayed input signal.
In accordance with yet another aspect of the invention, an apparatus is disclosed for predistorting an input signal to a non-linear amplifier. The apparatus comprises a series of correction elements. Each of the correction elements includes a predistortion calculation portion that determines an appropriate precorrection for a signal input into the correction element in accordance with an amplifier model. The amplifier model includes a limiting function that clips the input signal to reduce error due to amplifier saturation. Each also includes a difference element that applies the determined precorrection to the delayed input signal.
In accordance with still another aspect of the invention, an apparatus is disclosed for predistorting an input signal to a non-linear amplifier. The apparatus comprises a series of correction elements. Each of the correction elements includes a predistortion calculation portion that determines an appropriate precorrection for a signal input into the correction element in accordance with an amplifier model. The correction elements also include a delay portion that delays the input signal. Additionally, each includes a difference element that applies the determined precorrection to the delayed input signal.